Environmental Engineering Reference
In-Depth Information
a sandy clay and an upper clay. The associated soils are loams or clay loams, which are
poorly drained. Beneath the lake bed plains lies a diamicton or lower clay layer also histor-
ically described as a ground moraine. For this study, it was assumed that samples collected
from this lower clay unit, which is not exposed at the surface, represents the natural back-
ground conditions within the watershed. Consequently, the occurrence of metals within
the lower clay unit is due solely to nonanthropogenic causes. This is a reasonable assump-
tion since the unit is typically 3 m below the surface and isolated from surface activities by
at least 1.5 m of low-permeability clay (Rogers 1997b; Murray and Rogers 1999). Moreover,
because groundwater is not known to occur in this lower clay, there is negligible potential
of metals migrating from anthropogenic sources via groundwater pathways (Rogers 1996;
Rogers and Murray 1997). The location and distribution of the soil units just described are
shown in Figure 5. 20, and Figure 5.11 has a geologic cross section showing the distribution
of the lower clay unit. A series of diagrams (Figure 5.15a through h) depicts the evolution
of deposition of the glacial deposits within the watershed during the last 14,500 years.
The land use at each sampling location was well documented. Significant changes in
land use with time, however, may complicate the classification if, for example, redevelop-
ment changed a former industrial site into one zoned for commercial. Thus, in urban areas
such as Detroit, understanding the history of land use is important, and the changing
land use patterns present significant challenges in evaluating the occurrence and distri-
bution of metals in near-surface soil relative to land use. Although each soil sample was
categorized by its current land use classification (residential, commercial, and industrial),
every attempt was made through the use of aerial photography (black and white, both
digital and stereo pairs at a scale of 1:24,000 for years 1990, 1995, and 2000) obtained from
the Southeast Michigan Council of Governments (SEMCOG) to ensure that land use had
not changed substantially over the 10 year period of this study. In addition, a Phase I
environmental site assessment (Chapter 4) was conducted at each selected site. The Phase
I included evaluating and confirming historical land use at least 60 years prior to the date
of the investigation, and in many locations addressed a period over 100 years. Obviously,
significant changes in land use would tend to skew results. The inherent variability within
urban soils resulting from moving, backfilling, covering, and mixing was addressed by
collecting a spatially dispersed sample set (distributed across the watershed) over the
period from 1992 to 2002, and using aerial photography to evaluate changes in land use at
questionable sites.
Due to the differing nature of the various MDEQ investigations, not all samples were
analyzed for all parameters. Within the sample set, three metals, antimony, beryllium, and
thallium, were analyzed in less than 10% of the samples collected. Consequently, these
metals are reported separately in Table 9.3 along with their respective range of detection
levels. They have also been eliminated from the statistical results presented with the other
metals as their sample detection limits would have disproportionately skewed the results.
It is important to note that the range of detection for each of the low-occurrence metals
TABLE 9.3
Range of Concentration for Low-Occurrence Metals
Number of
Samples Analyzed
Range Detected
(mg/kg)
Metal
Antimony
7
3.7-6.1
Beryllium
6
0.5-1.5
Thallium
4
0.45-1.23
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